Add USDT to iocore/net, iocore/cache, http, http2

[moonchen]

Add USDT probes to the following:

• iocore/net: socket read/write, read/write disable, reenable, do_io_close, inactivity timeout, TLS read/write
• iocore/cache: read-while-writer attach, produce, starve, writer close
• proxy/http: tunnel producer/consumer/flow-control, add consumer, chunk decode, UA abort, transfer setup, background fill
• proxy/http2: send-window/write-buffer block, data frame, window update, RST_STREAM sent/received

The existing probes only mark one-shot lifecycle milestones; these cover the steady-state body-transfer and flow-control phases. They compile to no-ops unless built with -DENABLE_PROBES=ON.

[Copilot]

Pull request overview

This PR expands Apache Traffic Server’s USDT/SystemTap instrumentation beyond one-shot lifecycle probes by adding steady-state probes in hot-path data transfer and flow-control code across iocore (net/cache) and the HTTP/HTTP2 proxy layers. These probes are compiled out unless built with -DENABLE_PROBES=ON.

Changes:

• Add USDT probes for HTTP/2 flow-control and frame send/receive events (WINDOW_UPDATE, DATA, RST_STREAM, blocked conditions).
• Add USDT probes for HTTP tunnel and state-machine transfer/background-fill/abort/setup events.
• Add USDT probes for network I/O (socket + TLS read/write), enable/disable/reenable, inactivity timeouts, and cache read-while-writer lifecycle.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/proxy/http2/Http2ConnectionState.cc	Adds HTTP/2 probes for window updates, blocked sends, data frame sends, and RST_STREAM send/receive.
src/proxy/http/HttpTunnel.cc	Adds probes for tunnel producer/consumer handlers, chunk decoding, flow-control, and consumer attachment.
src/proxy/http/HttpSM.cc	Adds probes for transfer setup, background fill start/finish, UA abort, and server setup error.
src/iocore/net/UnixNetVConnection.cc	Adds probes for socket read/write, reenable, and do_io_close.
src/iocore/net/UnixNet.cc	Adds probe for inactivity timeout events (including default-timeout context).
src/iocore/net/SSLNetVConnection.cc	Adds probes for TLS read/write paths.
src/iocore/net/P_UnixNet.h	Adds probes for read/write disable transitions.
src/iocore/cache/CacheWrite.cc	Adds probes for read-while-writer writer produce/close events.
src/iocore/cache/CacheRead.cc	Adds probes for read-while-writer reader attach/starvation events.